Steel tanks with floating roof

The tanks with floating roof are most widespread method for decrease the loss when volatile oil   products are stored in them. They can be used when the tanks are open on top or they can be constructed inside in the tanks with fixed roofs. They can be used also when the tanks are closed and it is not admissible any product leak in the environment.

Floating roofs are strongly dependent on the good geometry of the shell, from vertical position of the anti-rotation device and the roof itself must not have a slope. The cases of discrepancy from the project condition of the tanks elements in the tanks lead to the key on the floating roof and to danger from fires. Big part from tanks with floating roofs that are in exploitation in Bulgaria are constructed in the end of 70 years and in the beginning of the 80 years. One of the most important problems is to find out whether these facilities are good for further use.

The main advantages of the tanks with floating roofs are as follow: 1. Minimizing of the loss of the stored product due to its evaporations. The factors influencing the product evaporation are: a) temperature of the liquid; b) the presence and the dimension of the vapor space above the liquid; c) the possibility for ventilating of the vapor space; d) the presence of the free surface of the product. 2. As a result of the decreased evaporating of the product the corrosion activity in the upper part shell courses is decreased too. 3. The safety from fire is increased which is a result of the decreased evaporation of the volatile stuff hence the lower risk from compound between the air and evaporating product which can be easily detonated. 4. Comparing with the tanks with fixed roofs there are smaller spaces inside the tanks with floating roofs which must be used for foam extinguish and it allows fitting fire extinguished installations with smaller power and their pipes to have smaller diameters.  5. The floating roof almost does not transmit the vertical longitudinal efforts to the shell. The risk from loss of stability is decreased.

The correct choice of the type of floating roof can decrease the risks during the exploitation. The factors which influence the tank construction and kind of the floating roof are as follow: - characteristic of the stored product; - climate conditions and conditions on the site; - volume of the stored product; - the requirements toward of the tank as a facility as a whole.

The floating roofs may be separated in the different groups depending on the different criteria. According to the presence of the fixed roof in the tank:

• external floating roofs. The floating roofs projected and used for open on the top tanks. They are made from steel and are welded. They must bear loadings from: a) weather conditions (rain, snow, wind); - 250 mm rain water for the period of 24 hours. With tank owner agreement the real values for geographical area can be accepted for calculating the rain loading; - the normal snow loading for Bulgaria is  s_t = 2,0 kN/m². The coefficient for snow overloading is accepted to be equal to γ_f =1,4. In this case the snow loading may be valid condition depend on area where the tank is situated. Snow load can be irregularly distributed on the roof. b) live loading on the roof; - 2,2 kN movable concentrated loading of two persons upon 0,1 m², who can stay on each one point on the roof, which is floating or is in the lower position upon its supports; c) loading  from rolling ladder and supporting truss; d) loading from additional manholes, nozzles, roof drain, and seal; e) loading of the product upon the roof; f) single deck roof – membrane and two neighboring compartments are punctured.    double deck roof  - two neighboring compartments are punctured. During this check, independently from the kind of the roof, there are not water and mobile load. - internal floating roof They appear because of the increased requirements of environment safety and to decrease the loss of oil product. These floating roofs are fit inside under the fixed roof itself. They can be fit even in the tanks with supported cone roof. They can be made from steel, aluminum, or synthetic materials. They must bear the loading from: a) 2,2 kN live concentrated load from two persons upon the 0,1m², who can stay upon any point on the roof, when it is floating or is in its lower position supported by its supports; b) if there are not other conditions, all project calculation must be done when the product density is 0,7t/m³. According to the kind of the construction, the floating roofs must be separated into the following groups.

• a) simplicity for execution, erecting and maintenance; Disadvantages: a) this kind of roof has not positive buoyancy and if its membrane be punctured it will sink; b) the wind can push the accumulated water upon the roof toward one board and it will cause the bending and probably sinking.

Criteria for determining the type of the floating roof The purpose of the tank, kind and the characteristics of the stored product, the conditions of the site must be taken into account when the type of floating roof is being chosen. The correct determination of its kind and the facilities connected with it can assures easer maintenance and quick return of the investments.

a) stability of the floating roof: It is one of the most important conditions for the correct work. Moving loads which are not balanced upon the roof can cause inclination of the roof and floating with slope. There is the higher risk for the roof to sink there. It is necessary to assure the work of the roof in the projected position by means of correct constructing; b) product - aggressiveness of the stored product. It is necessary to take care of chemical characteristic of the liquid and the possibility to damage the roof equipment which is not metal. Some chemical products can destroy the seals of the roof in several weeks of exploitation; - density of the product. According to the API Std. 650 volume weight of the product which are used when the calculations for the buoyancy are made is ρ_f  = 0,7 t/m³. It is not likely to store product with smaller density but it is possible to happen. The information for the minimal values of the ρ_f allows the designer to determine the sinking of the roof; - steam pressure of the product. It is one of the most important characteristic of the roof when the kind of the roof is being determined. According to the NFPA the use of the floating roofs in the tanks for storage of oil and oil products is recommended when the steam pressure of the liquid bigger or equal to 1,5 psia (78 mm Hg), but not bigger than 11,1 psia (570 mm Hg). If the steam pressure exceeds 11-12 psia the heating during the day will causes the considerable increase of the steams under the floating roof. The inflation of the central part as balloon is possible. The cooling of the product during the night will decrease the pressure and will allow to the roof to restore its previous form with slope toward the center. It is possible when there is the combination of the considerably inflated membrane by insider pressure, which slope is from the center toward the periphery and the water upon the roof, the correct drain of the water to be difficult. When the steam pressure of the liquid is high, approaching 11,1 psia or higher then it is better to use double deck floating roof. Its rigid construction keeps it in the projected form, even when there are the steam under it and it assure uninterrupted drain. On the other side the double deck roofs protect the roof from excessive heating. 

c) technological process

It is necessary to measure the speed of filling/emptying. When we do not consider this speed and it do not correspond to the erected technological facilities it is possible damage and even destroying of the roof. In order to fix the floating roof according to its designed position (for avoiding its rotation), the antirotation devices must be put inside in the tank, parallel to the shell. In the past the number of antirotation devices usually was two per tank. Now during the repairing works one of the antirotation devices can be removed and only one antirotation device can left. It must be done in order to neutralize of the inclinations of the autorotation devices from their designed position during the erections and during the exploitation. When the antirotation devices are not vertical it is possible to key of the floating roof with all consequences connected with it. It is necessary that the anti-rotation device supports all horizontal efforts which were supported before by two devices.

3. Constructive requirements a) approximately 200 mm free space must have between external floating roofs and the shell, where the seal will be mounted. The modern seals assure good jam and normal work when the distance between shell and roof is from 100 to 300 mm; b) minimal thickness of the steels in the pontoons in the membrane is 5mm (t_r ≥ 5 mm); c) the maximal distance between the roof supports of the floating roof must not be bigger than 1000 x t_r (thickness of the membrane); d) distance from roof supports to welding joint in the membrane is not smaller than 300 mm; e) when there are the girders stiffening the membrane, roof supports must be put in the area in this stiffening. f) the length of the supports is deferent depending on its status: - during the exploitation – minimal height must grant the tank filling without the liquid jet to touch the vertical board. It is unacceptable that the floating roof in its lowest position to lie upon the technological facilities; - during the repairing works on the tank - the minimal distance depends on considerations for easy doing of the repairing works. It is assumed to be 1800 mm in the lowest point of the roof. g) the floating roof , stepped through its supporting constructions on the bottom or when floats on the liquid must have the slope toward the roof drain not less than 1:100. For insider roofs this slope have not be considered; h) upon the floating roofs must be constructed  foam dam which height must not be less than 300 mm at 1 m from shell of the tank;

i) for fixing the roof according to the design conditions the antirotation devices must be considered. They do not allow the rotation of the roof. They must bear the horizontal forces, caused by the rolling ladder, by the uneven load from snow, by wind and by rotation of the stored product when it is mixed or the tank fills; j) the drain pipe must have the diameter not smaller  than 3” for the tank which have the diameter D ≤ 36 m.  The drain pipe must have the diameter not smaller than 4” for the tanks which have the diameter D > 36 m; k) the pontoons situated on the periphery must be separated by waterproofed barriers and every pontoon must have manhole for inspection;

l) in the central part of the roof must have one manhole for inspection, through which people can go to the roof when the roof is in its lower position and the tank is empty.